Currently used surgical sutures are available in various types made from both natural and artificial materials. In terms of shape, surgical sutures are classified as monofilaments (FIG. 4), braided multifilaments (FIG. 5) and twisted multifilaments (FIG. 6).
Surgical sutures made of synthetic polytetrafluoroethylene (PTFE) have recently been developed with a view to taking advantage of the low tissue interaction of PTFE and an example of such sutures is described in JP-B-61-34346. (The term "JP-B" as used herein means an examined Japanese patent publication.) Disclosed in JP-B-61-34346 is a surgical suture made of twisted or braided PTFE tapes or filaments that have a microstructure in which a number of fibrils oriented in a longitudinal direction are bound with small nodes at selected sites.
Commercially available porous PTFE sutures, on the other hand, have low tensile strength values that are smaller than those specified by the standards for plastic sutures in Notification No. 444 of the Ministry of Public Welfare of Japan (Extra Circular of Dec. 28, 1970). Examples of the diameters and tensile strength values of the commercial products and an extract of the Notification No. 444 are shown in Table 1 below. Further, these commercial sutures have fine asperities on the surface as shown in FIG. 7.
TABLE 1 ______________________________________ Standards in Commercial product Notification No. 444 Tensile Tensile Diameter strength Diameter strength (mm) (kg) (mm) (kg) ______________________________________ 0.221 1.10 0.203-0.245 .gtoreq.1.13 0.163 0.62 0.152-0.203 .gtoreq.0.68 0.132 0.43 0.102-0.152 .gtoreq.0.45 ______________________________________
The suture shown in JP-B-61-34346 is composed of twisted tapes or filaments, and thus it has the following two major problems because of twisting: (1) it has low porosity and elasticity, and (2) the surface asperities reduce the slipping property of the suture and increase the chance of adversely affecting the tissues of wounds or surgical incisions.
The commercial porous PTFE sutures have a problem in that they do not have a sufficient tensile strength to satisfy the standards specified by the Ministry of Public Welfare of Japan in Notification No. 444.